Among various types of displays, liquid crystal displays (LCDs) adopting liquid crystal have been most commonly used thanks to low power consumption. There have been two types of display modes and driving methods of the liquid crystal displays, namely a passive matrix system and active matrix system. In an age of multimedia, there have been growing demands for the improvements in resolution, contrast, gradation (multicolor, full color), and angle of view of displays. Considering that the passive matrix system is hardly capable of meeting the demands, the active matrix system has been contrived. In this system, a switching element (active element) is provided in each pixel so that the number of scanning electrodes to be driven is increased. In these days, the improvements in resolution, contrast, gradation, and angle of view have been achieved by adopting the active matrix system. An active matrix liquid crystal display is arranged in such a manner that pixel electrodes provided in a matrix manner are electrically connected to scanning lines provided around the pixel electrodes, through the intermediary of active elements. Examples of the active elements includes a two-terminal nonlinear element and three-terminal nonlinear element, and a currently dominating type of the active elements is a three-terminal thin-film transistor (hereinafter, TFT).
In recent times, mobile devices typified by mobile phones are commonly equipped with liquid crystal displays adopting the active element system. Such mobile devices are required to have good visibility both in the open air and in doors. Conventional transmission liquid crystal displays can reproduce images with high quality in doors, but the quality of the images significantly deteriorates when used in intense outside light. To reproduce images with sufficient quality in all kinds of settings both in and outside of doors, a system (front-light system) in which an auxiliary light source is provided in a reflective liquid crystal display and a semi-transmissive liquid crystal display in which each pixel has a reflective display area and transmissive display area have actively been developed.
In the field of mobile devices such as mobile phones, devices having two displays, namely a display for main information and a display for simple information, have become mainstream. The display for main information adopts an active matrix panel in order to display large-sized data such as visual images. The display for simple information has a screen smaller than that of the display for main information and displays small-sized data such as a clock, and thus adopts a passive matrix panel. The double-sided display is realized by disposing these liquid crystal displays in a back-to-back manner. However, the use of two liquid crystal displays has been accompanied with various problems typified by the impediment of the reduction of thickness, weight, and costs.
To overcome these problems, Japanese Laid-Open Patent Application No. 2000-193956 (Tokukai 2000-193956; published on Jul. 14, 2000) discloses a liquid crystal display which realizes double-sided display by incorporating one liquid crystal display with two reflective polarizers.
FIG. 15 shows a liquid crystal display 110 of this patent application. In the liquid crystal display 110, a first reflective polarizer 118 is provided on one side of a liquid crystal cell 122, and a second reflective polarizer 142 is provided on the opposite side of the liquid crystal cell 122. Furthermore, on the outer surface of the first reflective polarizer 118, i.e. on the surface not facing the liquid crystal cell 122, a first absorbing polarizer 114 is disposed. Between the liquid crystal cell 122 and second reflective polarizer 142, a scattering layer 138 is disposed. On the outer surface of the second reflective polarizer 142, a second absorbing polarizer 146 is disposed, and on the outer surface of the second absorbing polarizer 146, a light absorbing layer 150 is disposed in a detachable manner. Further, on the outer surface of the first absorbing polarizer 114, a substantially transparent light guiding plate 152 is provided for emitting light toward the liquid crystal cell 122, and light sources 166 are provided for emitting light towards the light guiding plate 152.
The directions of the following axes are determined as illustrated in FIG. 5: a transmission axis 114T and absorption axis 114A of the first absorbing polarizer 114, a transmission axis 118T and reflection axis 118R of the first reflective polarizer 118, a transmission axis 142T and reflection axis 142R of the second reflective polarizer 142, and a transmission axis 146T and absorption axis 146A of the second absorbing polarizer 146.
With the arrangement above, in a dark place, the liquid crystal display 110 can operate as a reflective display viewed from the outer surface of the light guiding plate 152, by causing the light emitted from the light sources 166 to be reflected on the second reflective polarizer 142. On the contrary, in a dark place, the liquid crystal display 110 can also function as a transmissive display viewed from the outer surface of the second reflective polarizer 142, by causing the light emitted from the light sources 166 to pass through the second reflective polarizer 142 and second absorbing polarizer 146.
However, since the liquid crystal display 110 of Japanese Laid-Open Patent Application No. 2000-193956 performs double-sided display using the reflective polarizers for both the front and back displays, the following problems occur: In a well-lighted place, when the liquid crystal display 110 is used as a transmissive display and viewed from the outer surface of the second reflective polarizer 142, the direction of polarizing the linearly polarized light incident on the liquid crystal cell 122 from the light guiding plate 152 is altered for 90 degrees in the case of the arrangement in FIG. 15, for carrying out black display. In this case, if outside light enters from the outer surface of the second reflective polarizer 142, the light incident on the liquid crystal cell 122 is reflected on the first reflective polarizer 118, so as to return towards the outer surface of the second reflective polarizer 142. For this reason, the black display is not properly carried out so that the quality of displayed images deteriorates.
In this manner, the conventional double-sided liquid crystal display 110 cannot properly carry out black display only either in a dark place or in a well-lighted place.
The present invention is done to solve the above-identified problem. The objective of the present invention is to provide (i) a double-sided display which can properly carry out black display both in a dark place and in a well-lighted place, and (ii) a device including the same.